In recent years, in the field of information recording, studies on optical information recording methods have been pursued in various places. Such optical information recording methods have advantages of being able to record and reproduce in a noncontact way and deal with read-only type, write-once type, and rewritable type memory systems and are expected to be widely used from industrial use to consumer use as methods capable of realizing inexpensive large-capacity files.
The increasingly large capacity of optical recording media (hereinafter, also referred to as “optical disks”) for the above various optical information recording systems has mainly been attained by shortening the wavelength of the laser light serving as the light source used in optical information recording systems and by adopting an objective lens having a high numerical aperture to make the spot size on the focal surface smaller.
For example, in a CD (compact disk), the laser light wavelength is 780 nm, the numerical aperture (NA) of the objective lens is 0.45, and the capacity was 650 MB, while in a DVD-ROM (digital versatile disk read-only memory), the laser light wavelength is 650 nm, the NA was 0.6, and the capacity is 4.7 GB.
Further, for next generation optical disk systems, studies are being made on increasing capacity by using an optical disk formed with a thin light transmitting protective film (cover film) of, for example, 100 μm or so on an optical recording layer, irradiating laser light for recording/reproduction from the protective film side, making the laser light wavelength 450 nm or less, and making the numerical aperture (NA) of the objective lens 0.78 or more.
Also, in recent years, development of a rewritable type multilayer optical disk using a phase change type recording material and having two optical recording layers has been pursued. Hereinafter, an optical disk having a plurality of optical recording layers will also be referred to as a “multilayer optical disk”, while an optical disk having one optical recording layer will also be referred to as a “single layer optical disk”.
The present inventors have been engaged in development of a phase change type multilayer optical disk and disclosed results at the Optical Data Storage (ODS) Symposium in 1999 and ODS Symposium in 2001.
A phase change optical disk, whether a single layer optical disk or a multilayer optical disk, requires a process called “initialization” before shipping to the market.
In a production process of a phase change type optical disk, generally, a film of a phase change type recording material is formed on a substrate made of polycarbonate etc. by a sputtering apparatus. In the “as-deposited” stage after forming the film, the phase state of the phase change type recording material is close to an amorphous state.
In a phase change optical disk, when recording information, the phase state of the phase change type recording material is required to be a crystalline state before the recording. The process of changing the amorphous state right after forming the film to the crystalline state is called as the “initialization process”.
In the currently widely used initialization apparatus, the entire surface of the optical recording layer is crystallized by focusing laser light on the optical recording layer to be initialized to heat the phase change type recording material and scanning the entire surface of the optical recording layer.
At this time, the beam shape of the laser light focused on the optical recording layer has a beam width of, for example, about 1 μm in the disk rotation direction and about 100 μm in the radius direction.
When initializing the optical recording layer on the multilayer optical disk having a phase change type optical recording layer provided as the first layer from the light incident side by using such an initialization apparatus, as described in Japanese laid open patent application (Kokai) No. 2001-250265, it is known that due to light interference generated by uneven thickness of an interlayer between a first optical recording layer and a second optical recording layer, a change of the light intensity of the initializing light occurs on the first recording layer and causes uneven initialization.
Japanese laid open patent application (Kokai) No. 2001-250265 proposes to use a laser of a wavelength near 810 nm and a material of the interlayer having absorption in that wavelength and not having absorption in a wavelength near 400 nm. As an example, it describes to mix a pigment material having absorption in a wavelength near 810 nm into the material of the interlayer.
However, if actually mixing the above pigment material into the material composing the interlayer, the material of the interlayer and the pigment material sometimes interact and absorption occurs at a wavelength near 400 nm.
The light absorption spectrum of the above pigment material alone and the light absorption spectrum in case of mixing the above pigment material into the material of the interlayer are shown in FIG. 1A and FIG. 1B. Mechanical properties, thermal properties, and other properties suitable for an optical disk have to be sought from the material of the interlayer in addition to the light absorption properties such as in FIG. 1A.
Development of a material satisfying even the light absorption properties may be achieved with further development work, but is not easy.